1. Field of the Invention
The present invention relates to a projection type image display system, and more particularly, to a projection type color scrollable image display system.
2. Description of the Related Art
In conventional projection type image display systems which deliver image information to people, a light valve, such as a liquid crystal display (LCD) or a Digital Micro-mirror Device (DMD), is used to perform switching for high-speed information processing. The light valve controls the on/off operation of light emitted from a light source (e.g., a high output lamp) on a pixel-by-pixel basis and forms a picture. A magnifying projection optical system provides the picture to a large screen. Projection type image display systems are classified into either 3-panel projection type image display systems or single-panel projection type image display systems, according to the number of light valve panels used. Projection type image display systems try to provide a high luminance for a large screen while using a single panel to overcome a complicated and expensive optical system.
In a projector adopting a conventional single-panel color image display system, white light radiated from a white light source is separated into R, G, and B color beams using a color wheel, and the three color beams are sequentially transmitted to a single light valve. The light valve operates and creates images according to the sequence of color beams received.
These general single-panel color image display systems have smaller optical systems than three-panel projection image display systems, in which different colors are obtained using an optical separation/combination system and images of different colors are created using three light valves. However, these single-panel color image display systems provide only ⅓ of the optical efficiency of the three-panel color image display systems because color wheels are used.
As described above, a single-panel color image display system using a color wheel provides only ⅓ of the light efficiency of a three-panel image display system. A color scrolling method has been designed to increase the optical efficiency of a single-panel color image display system.
According to the color scrolling method, white light is separated into R, G, and B color beams, and the three color beams are sent simultaneously to different locations on a light valve to form R, G, and B color bars. Since an image cannot be produced until each of the R, G, and B color beams reach all pixels of the color areas in the light valve, the color bars are moved at a constant speed by a color scrolling means. The use of the color scrolling method enables a single-panel color image display system to have the light efficiency of a three-panel color image display system.
FIG. 1 shows a single-panel scrolling color image display system disclosed as in U.S. Patent Publication No. 2002/191154 A1. As shown in FIG. 1, white light emitted from a lamp type light source 102 passes through first and second lens arrays 104 and 105 and a polarization conversion system (PCS) 106 and is condensed by a condenser lens 107. The white light is separated into R, G, and B color beams by first through fourth dichroic filters 108, 110, 112, and 114, and the R, G, and B color beams are recombined.
To be more specific, first, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 108 and advance along a first light path L1, while the blue beam B is reflected by the first dichroic filter 108 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 110. The second dichroic filter 110 transmits the red beam R along the first light path L1 and reflects the green beam G along a third light path L3.
The blue beam B and the green beam G that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 112, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 114. The combined R, G, and B beams are transmitted by a polarization beam splitter (PBS) 128 and are made incident upon a light valve 130. Reference numerals 126 and 132 denote a polarizer and an analyzer, respectively.
First through third prisms 120, 116 and 118 are disposed on the first through third light paths L1, L2, and L3, respectively, and rotate at a uniform speed such that R, G, and B color bars are formed on the light valve 130 and scrolled.
As described above, in a conventional single-panel color image display system, while color separation and color combination are being performed using the first through fourth dichroic filters 108, 110, 112, and 114, the first through third prisms 120, 116, and 118 are rotated to achieve color scrolling.
The scrolling of the R, G, and B color bars due to rotation of the first through third prisms 120, 116, and 118 is shown in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 120, 116, and 118 corresponding to colors are synchronously rotated.
A color image is obtained by processing image information for each of the pixels of the light valve 130 in synchronization with a motion of the color bars. The color image is magnified by a projection lens 134. Then, the magnified image is made incident on a screen.
A conventional single-panel color image display system adopting such a scrolling technique uses different light paths for each color and then recombines the separated beams. The combined beams are sent to the PBS 128 via relay lenses which are installed on the light paths.
Hence, the optical system becomes bulky, and the manufacture and assembly thereof is complicated.
Also, since the three prisms 120, 116, and 118 are separately rotated to perform color scrolling, it is difficult to synchronize this rotation with the driving of the light valve 130.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. The conventional color image display system must synchronize the light valve 130 with the three prisms 120, 116, and 118 in order to achieve scrolling. However, controlling the synchronization is not easy. Due to the circular motion of the scrolling prisms 120, 116, and 118, the color scrolling speed by the three scrolling prisms may be irregular, consequently deteriorating the quality of the resultant image.
Three motors for rotating the three scrolling prisms 120, 116, and 118 generate a lot of noise during operation. Additionally, a color image display system utilizing three motors is manufactured at a greater cost than a color wheel type color image display system which utilizes a single motor.